The International Astronomical Union stirred up quite a controversy a few years ago when it dropped Pluto from the rank of major planets. For astronomers, though, disagreements over what to call things are nothing new. They've been arguing about names for centuries.

An example is the largest moons of Jupiter. Galileo Galilei discovered them 400 years ago when he first looked at Jupiter through his telescope. He published his discovery in a book a couple of months later, and it made him famous.

Galileo wanted to use that fame to honor his patrons, the Medici family. He offered the family a choice of "Medicean" moons or "Cosmic" moons -- the latter in honor of family leader Cosimo the Second.

Galileo didn't give the moons individual names -- just numbers. A few years later, another astronomer came up with three different ways to name the moons -- including a system that would honor his patron, the Duke of Brandenburg. And Johannes Kepler suggested naming them after four of Jupiter's mythological lovers.

All of the suggestions were basically ignored, with astronomers generally using Galileo's system of numbers.

But as they discovered a bunch of moons of Saturn, they decided they needed a more formal system for naming moons. So in the 1800s, they went back to Kepler's idea from more than 200 years earlier. The four big moons of Jupiter were named Io, Europa, Ganymede, and Callisto -- names that are still used today.